While there is some commonality between gears and screw rotors, a major difference is in the fluid sealing requirements of screw rotors. As in the case of gears, screw rotors have pitch circles which represent locations of equal tangential velocity for conjugate pairs of rotors. The spiral grooves in the rotors are the locations of the volumes of gas which are trapped and compressed due to the coaction of a conjugate pair of rotors and an enclosing casing. Accordingly, the volumes of the spiral grooves are a major design consideration with their width, depth, length and number being design variables. The shape of the cross section of the spiral grooves includes the variables of width and depth as well as the shape requirements for the driving/driven coaction between the conjugate pair of rotors. Additionally, the conjugate pair must meet the sealing requirements as the line contact advances along the rotor profile in the driving/driven coaction and as the rotor tips and end faces coact with the enclosing casing. This line contact follows the perimeters of the rotor profiles and is therefore at a varying tangential speed and has significant radial components. Additionally, the shape and cross section of the spiral grooves must meet requirements for ease of manufacture and cutting tool life. One problem associated with conventional screw rotor designs is that the pressure angle and lobe thickness are interrelated. It is desirable to minimize the pressure angle, the angle of contact between the rotors in the contact zone near or at the pitch circle, to provide reduced contact loading. However, the reducing of the pressure angle has an attendant undesirable reduction in lobe thickness such that conventional designs represent a compromise between desired pressure angle and desired lobe thickness.
Assuming that each respective lobe tip of each rotor is in tangential contact with a root of the other rotor during a point in each revolution, the addendum of the lobes of one rotor will be coincident to the dedendum of the lobes of the other rotor as measured along a line connecting the centers of the two rotors. Ignoring running clearances, machining tolerances, wear, thermal expansion, etc. there are three nominal points of tangency between a conjugate pair of rotors, namely between the pitch circles and between the tip circle of each rotor and the root circle of the other rotor.